As examples of tubes for fluid transfer may be mentioned hose for transporting petrol (synonymously“gasoline”), and in particular for conveying the petrol from the tank to the engine of a motor vehicle. As other examples of fluid transfer may be mentioned: the fluids involved in a fuel cell, the CO2 system for cooling and air conditioning, the hydraulic lines, the cooling circuit, other air conditioning systems and for intermediate pressure power transfer.
For safety and environmental protection reasons, motor-vehicle manufacturers require these tubes to have both mechanical properties such as strength and flexibility with good cold (−40° C.) impact strength as well as good high-temperature (125° C.) strength, and also very low permeability to hydrocarbons and to their additives, particularly alcohols such as methanol and ethanol. These tubes must also have good resistance to the fuels and lubrication oils for the engine. These tubes are manufactured by coextruding the various layers using standard techniques for thermoplastics.
Among the characteristics of the specification for petrol tubes, five are particularly difficult to obtain jointly in a simple manner:                cold (−40° C.) impact strength—the tube does not break;        fuel resistance;        high-temperature (125° C.) strength;        very low permeability to petrol;        good dimensional stability of the tube in use with the petrol.        
In multilayer tubes of various structures, the cold impact strength remains unpredictable before having carried out the standardized tests for cold impact strength.
Moreover, it is already known from Patent Application EP 0 781 799 that in motor vehicles, under the effect of the injection pump, the petrol flows at high speed in the pipes connecting the engine to the tank. In certain cases, the friction between the petrol and the internal wall of the tube can generate electrostatic charges, the accumulation of which may result in an electrical discharge (a spark) capable of igniting the petrol with catastrophic consequences (an explosion). It is therefore necessary to limit the surface resistivity of the internal face of the tube to a value of generally less than 106 ohms/square. It is known to lower the surface resistivity of polymeric resins or materials by incorporating conductive and/or semiconductive materials into them, such as carbon black, steel fibres, carbon fibres, and particles (fibres, platelets or spheres) metallized with gold, silver or nickel.
Among these materials, carbon black is more particularly used, for economic and processability reasons. Apart from its particular electrically conductive properties, carbon black behaves as a filler such as, for example, talc, chalk or kaolin. Thus, those skilled in the art know that when the filler content increases, the viscosity of the polymer/filler blend increases. Likewise, when the filler content increases, the flexural modulus of the filled polymer increases. These known and predictable phenomena are explained in “Handbook of Fillers and Reinforcements for Plastics”, edited by H. S. Katz and J. V. Milewski—Van Nostrand Reinhold Company—ISBN 0-442-25372-9, see in particular Chapter 2, Section II for fillers in general and Chapter 16, Section VI for carbon black in particular.
As regards the electrical properties of carbon black, the technical report “Ketjenblack EC—BLACK 94/01” by Akzo Nobel indicates that the resistivity of the formulation drops very suddenly when a critical carbon black content, called the percolation threshold, is reached. When the carbon black content increases further, the resistivity rapidly decreases until reaching a stable level (plateau region). It is therefore preferred, for a given resin, to operate in the plateau region in which a metering error will have only a slight effect on the resistivity of the compound.
Polyamide- and EVOH-based tubes for transporting petrol are also known from Patent Application EP 0 731 308. These tubes may have a four-layer structure comprising, respectively, a PA-12 outer layer, a binder layer, which is a grafted polyolefin, an EVOH layer and an inner layer in contact with the petrol, comprising a blend of a polyamide and a polyolefin having a polyamide matrix.
Patent EP 428833 describes a three-layer tube comprising, respectively, a PA-12 outer layer, a binder layer which is a grafted polyolefin and an EVOH inner layer in contact with the petrol.
Patents EP 428834 and EP 477606 describe a five-layer tube comprising, respectively, a PA-12 outer layer, a binder layer which is a grafted polyolefin, a PA-6 layer, an EVOH layer and a PA-6 inner layer in contact with the petrol.
U.S. Pat. No. 5,038,833 describes a three-layer tube comprising, respectively, a PA-12 outer layer, an EVOH layer and a PA-12 inner layer in contact with the petrol.
All these tubes have good properties but the thickness of the binder layers is not easy to control and as a result, there may be delaminations. In the tube described in U.S. Pat. No. 5,038,833, there is no binder but delaminations do occur.
A multilayer tube comprising a polyamide outer layer, a copolyamide binder layer, an EVOH layer, another copolyamide binder layer and a polyamide inner layer in contact with the petrol respectively, has now been found. The copolyamide binder is particularly effective and is also easy to coextrude.